Non-linear filter for dml

ABSTRACT

A circuit is disclosed having a component having repeatable distortion characteristics; and a drive circuit for providing a drive signal and comprising a non-linear filter for pre-compensating for distortion introduced by the component having repeatable distortion characteristics in response to the drive signal, the distortion having a non-linear response to the drive signal.

This application claims the benefit of U.S. Provisional PatentApplication No. 61/984,621, filed on Apr. 25, 2014, the entire contentsof which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to communication and more particularlyto optical communication.

BACKGROUND

Optical transmitters employing Directly Modulated Lasers (DML) such asVertical Cavity Surface Emitting Lasers (VCSELs) are rated to operate upto a predetermined data rate. Problematically, when operating at higherdata rates, distortion from the DML itself limits performance of thedevice and thus the data link. The DML transmits an optical signal thatdiffers from the drive signal provided thereto such that signalreception is substantially affected beyond short transmission distances.Added jitter and vertical eye closure from distortion introduced byVCSEL can cause significant reduction in signal-to-noise ratio (SNR).These limitations on performance place a limit on the transmissiondistances for higher data rates.

Linear filters are used conventionally to partially compensate for thedistortion due to the DML itself However, linear filters fail to achieveoptimal compensation for the distortion. It would be advantageous toovercome some of the shortcomings of the prior art.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In accordance with an aspect of at least one embodiment there isprovided a component having repeatable distortion characteristics; and adrive circuit for providing a drive signal and comprising a non-linearfilter for pre-compensating for distortion introduced by the componenthaving repeatable distortion characteristics in response to the drivesignal, the error having a non-linear response to the drive signal.

In accordance with an aspect of at least one embodiment there isprovided a method comprising: providing a drive current for driving aDirectly Modulated Laser (DML); filtering the drive current with anon-linear filter to provide pre-compensated drive currentpre-compensated for errors in a signal resulting from driving the DMLwith the drive current, wherein an output signal from the DML inresponse to the pre-compensated drive current better approximates thedrive current to incur reduced errors.

In accordance with an aspect of at least one embodiment of the inventionthere is provided a circuit comprising: an input port for receiving afirst signal; a plurality of taps, each tap comprising an input port forreceiving a tap input signal, a first input port for receiving a firstweight, a second input port for receiving a second other weight, and abiasing circuit for biasing an applied weighting between the firstweight and the second weight to bias the tap signal, the biased tapsignal for modifying the first signal.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a typical optical output signal amplitude of a DML(VCSEL) in response to a direct driving input signal.

FIG. 2 is a simplified block diagram of a linear finite impulse response(FIR) filter.

FIG. 3 is a logic diagram of non-linear FIR filter.

FIG. 4 is a diagram of a non-linear FIR filter implementation.

FIG. 5 is a diagram of another non-linear FIR filter implementationoptimized for performance.

FIG. 6 is an eye diagram of an unfiltered drive signal alongside an eyediagram of an output signal corrected with a non-linear FIR filter suchas that of FIG. 4 or FIG. 5.

FIG. 7 is a graphical representation of the transmit signal before andafter filtering with a 4 tap non-linear FIR filter.

FIG. 8A shows a sample circuit for implementing a non-linear filter forpre-compensating a drive signal for driving a directly modulated laser(DML).

FIG. 8B shows another sample circuit for implementing a non-linearfilter for pre-compensating a drive signal for driving a directlymodulated laser (DML).

FIG. 8C shows another sample circuit for implementing a non-linearfilter for pre-compensating a drive signal for driving a directlymodulated laser (DML).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following description is presented to enable a person skilled in theart to make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the scope ofthe invention. Thus, the present invention is not intended to be limitedto the embodiments disclosed, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Referring to FIG. 1, shown is a typical optical output signal amplitudeof a Directly Modulated Laser (DML) in the form of a Vertical CavitySurface Emitting Laser (VCSEL) in response to a direct driving inputsignal. As is evident, the optical signal generated (thin line) fails tofollow accurately the signal provided (thick line). The resultingovershoots and undershoots add distortion to the signal. The distortionappears as both jitter affecting the width of the eye and amplitudevariations affecting the opening of the eye.

Referring now to the eye diagram that is shown on the left-hand side ofFIG. 6, the signals are distorted horizontally—i.e., increasedjitter—and vertically—i.e., degraded SNR. As is evident, the inner eyeopening is significantly smaller than it would be if the signal wasundistorted. Correction of these distortion artifacts within thetransmit signal are important to enable transmission of higher datarates over longer distances.

The distortion artifacts resulting from DML optical response areamplitude dependent and thus non-linear in nature. The rising edge andfalling edge responses are different and they each need to becompensated differently. Further, compensating one edge response mayadversely affect the other edge or may fail to achieve significantimprovement without compensating for the other edge as well. Thus,conventional approaches using linear filters for compensating for thedistortion from the DML response are not optimal.

A second problem is implementation efficiency. If the distortion isrepeatable and calculable, it may be possible using a DSP to reduce thenonlinear distortion within the DML signal; that said, such animplementation would be costly and would not lend itself to inexpensive,low power and compact implementation. A more simple non-lineardistortion reduction method would be preferred.

Referring now to FIG. 2, shown is a simplified block diagram of atypical linear finite impulse response (FIR) filter. A signal isprovided to the filter and is then summed with a weighted delayedversion of the signal, termed as a delayed tap, or a plurality ofweighted sequentially delayed versions of the signal or delayed taps.Linear FIR filters are well known and well studied.

Because the distortion is non-linear in nature, a linear filter is notsuitable to addressing the distortion concerns completely. In fact, sucha linear filter, will fail to substantially correct the problemsdisclosed above, reducing distortion in one of the rising or fallingedge response while compounding the distortion in the other.

Referring now to FIG. 3, shown is a non-linear FIR filter design forproviding pre-compensation for some of the non-linearities shown inFIG. 1. Here, a signal is provided to the filter consisting of multipledelayed taps and each tap's contribution is weighted by two differentfactors dependent on input signal's instantaneous amplitude, resultingin an amplitude-dependent, non-linear filter response. The filtergenerates a different response to each of the rising and falling edgesand approximately compensates for the non-linear response of the DML inresponse to a driver signal. The use of two weights per tap, combinedwith scaling the tap contributions with instantaneous input signalamplitude, allows for non-linear filter response. While the use of theFIR architecture supports compact and efficient implementation. Forexample, the non-linear FIR filter shown is implementable as an analoguecircuit within a semiconductor, for example, without relying on complexprocessing circuitry such as a DSP.

Referring now to FIG. 4, shown is a simplified diagram of a practicalimplementation of non-linear FIR filter for Non-Return to Zero (NRZ)signaling. Again, a signal provided is delayed and tapped, and each tapcontribution is weighted differently, depending on input signal level ofone or zero, to provide level dependent non-linear operation. The tapcontributions are added back into the signal to provide filteringthereof. Signal scalars are reduced to gates in the filter shown, assuits an integrated hardware implementation.

Referring now to FIG. 5, shown is a diagram of another non-linear FIRfilter implementation optimized for performance in the presentembodiment. Here again, each tap signal is acted on by two differentweights. A multiplexer is used to select the weighting formultiplication. The input signal level is used to select one of themultiplexer's input weights, and the multiplier scales the tap signal asper selected weight for each stage. Since the weights for each tapswitch as per the input signal's level, the filter output need notfollow a linear contour. The resulting non-linear filteringpre-compensates for distortion in the DML.

Referring again to FIG. 6, shown is an eye diagram of an uncorrectedoutput signal (left-hand side) alongside an eye diagram of an outputsignal corrected with a non-linear FIR filter such as that of FIG. 4 orFIG. 5 (right-hand side), sand according to the present embodiment. Asis shown, the eye has opened up considerably with reduced jitter andimproved SNR. An improved eye diagram is typically reflective ofimproved ability to transmit over greater distances and reduced error insignal reception.

FIG. 7 shows a graphical representation of the transmit signal before(left-hand side) and after filtering with a 4 tap non-linear FIR filter(right-hand side). Most noteworthy, signal distortion is greatly reducedafter a short time reducing distortion central to the eye. At the risingedge and falling edge, distortion remains, but it is significantlyreduced. Thus, the eye opening in an eye diagram is improved. Further,other frequency components resulting from the distortion are reducedwith reduced distortion.

Just looking to the falling edge, it is seen that whereas withoutfiltering, the signal bounces at the bottom down and up, with filteringthe signal remains substantially in alignment with the desired signalcontour. On the rising edge, two notable bounces are reduced to onesmaller bounce, thereby limiting the effect of the bounce on the top ofthe eye.

FIGS. 8A-C shows three sample circuits for implementing a non-linearfilter according to the embodiment. Each circuit has different drawbacksand advantages, but effectively, the filter design allows not only foranalogue hardware implementation, but for varied implementation to takeadvantage of different power sources, power levels, and other designcriteria. Architectures supporting implementation flexibility aretypically desirable as they are useful in many different applicationsand well suited to implementation in many different devices.

As is seen in each of the circuit diagrams, two currents proportional toweights are shown designated with “w” (w_0 and w_1) being multiplexedinto the scaling circuit for each tap determined by level of the inputsignal (Dp, Dn). Alternatively, the currents proportional to weights areapplied to a scaling circuit such that they are first scaled by theinput signal (Dp, Dn) followed by the tap signal (Tnp, Tnn). Furtheralternatively, currents proportional to the weights are applied to ascaling circuit where the signals being scaled are a logical combinationof input signal (Dp, Dn) and the tap signal (Tnp, Tnn). The logicalcombinations include input signal (Dp, Dn) logically OR'd with tapsignal (Tnp, Tnn) designated as “Dp+Tnp”; and input signal (Dp, Dn)logically AND'd with tap signal (Tnp, Tnn) designated as “Dp.Tnp”. Thescaled version of these logically combined signals in current form isthen summed through a wire OR to produce a single tap contribution thatis dependent on the weights and the input signal amplitude. Multiple tapcontributions are summed to generate a resulting signal that has anamplitude dependent non-linear characteristic.

Though FIGS. 8A-C show one tap for each architecture, it is understoodby those of skill in the art that any number of taps is supported andselection of a number of taps is dependent upon the circuit designrequirements. Further, though two weights are shown, the filterarchitecture described above may be implemented with additional weightsto correct for more complex amplitude dependent non-linear effectsrequiring higher granularity or resolution in amplitude levels.

Though the above embodiments are directed to pre-compensating the drivecurrent, filtering of received signals to improve data detection is alsosupported. The general architecture for non-linear filter as shown inFIG. 3 can be used for a received signal. In end-to-end fibre opticcommunications such as fibre optic cables for communicating, thetransmitter and receiver pairing is known and the weights within thereceiver are tuned for use with a specific receiver or are adjustedbased on a transmitter from which a signal is received.

In use, a circuit is designed and manufactured. Once manufactured, thecircuit is tested with a representative DML component and based on thecombined circuit and DML transmit signal characteristics, the non-linearFIR filter weights are adjusted to pre-compensate the drive current forthe DML. Thus, each product is compensated individually, accounting forknown DML response issues as well as circuit specific response issuesfor a given DML. Once compensated, the circuit operates in compensatedmode. Optionally, the circuit's operating parameters are readjusted tore-compute the weights for the non-linear FIR filter at intervals.

In another embodiment, the optical output signal is tapped and providedas feedback to the transmit circuit where the non-linear FIR filter isadjusted in response to changes in performance of the DML output signal.Further optionally, the circuit is designed and manufactured with fixedweighting for the non-linear FIR filter.

In another embodiment, the manufactured devices are tested, thenon-linear FIR filter is tuned—weights are set—and the circuit is testedagain. Based on its performance, the circuit is assigned a qualitylevel. Thus, some manufactured drive circuits support 25 GHz whileothers support only 15 GHz—determined after tuning in the manufacturingstage. This allows for a more coarse tuning process with the performanceassignment then dividing between circuits with best tuning and thosewith less effective tuning results.

Numerous other embodiments may be envisioned without departing from thescope of the invention.

What is claimed is:
 1. A circuit comprising: a component havingrepeatable distortion characteristics; and a drive circuit for providinga drive signal and comprising a non-linear filter having at least a tapfor pre-compensating for distortion introduced by the component havingrepeatable distortion characteristics in response to the drive signal,the distortion having a non-linear response to the drive signal.
 2. Acircuit according to claim 1 wherein the component comprises a DirectlyModulated Laser (DML).
 3. A circuit according to claim 2 wherein the DMLcomprises a Vertical Cavity Surface Emitting Laser (VCSEL).
 4. A circuitaccording to claim 2 wherein the DML comprises a Distributed FeedBack(DFB) laser.
 5. A circuit according to claim 3 wherein the DML isoperated at at least 25 Gbps.
 6. A circuit according to claim 4 whereinthe non-linear filter comprises a non-linear Finite Impulse Response(FIR) filter having at least 2 weights for application at each delayedtap and supporting at least one delayed tap.
 7. A circuit according toclaim 4 wherein the non-linear filter comprises a non-linear FiniteImpulse Response (FIR) filter having at least 2 weights for applicationat each delayed tap and supporting at least 3 delayed taps.
 8. A circuitaccording to claim 1 wherein the non-linear filter comprises anon-linear Finite Impulse Response (FIR) filter having at least 2weights for application at each delayed tap and supporting filtering ofboth a rising edge, low to high signal level response and a fallingedge, high to low signal level response.
 9. A circuit according to claim8 comprising: for each tap a first input port for receiving a firstweight, a second input port for receiving a second other weight, aswitch for switching between the first weight and the second weight, anda weighting circuit for weighting of a signal within the tap to producea tap output, tap output signals from different taps combined to formthe drive signal.
 10. A circuit according to claim 8 comprising: foreach tap a first input port for receiving a first weight, a second inputport for receiving a second other weight, a scaling circuit for scalingthe first weight and the second weight, and a weighting circuit forweighting of a signal within the tap to produce a tap output, tap outputsignals from different taps combined to form the drive signal.
 11. Acircuit according to claim 1 wherein the non-linear filter comprises anon-linear Finite Impulse Response (FIR) filter having greater than 2weights at each delayed tap supporting filtering of a complex amplitudedependent non-linear distortion for a signal with a modulation schemehaving greater than 2 amplitude levels of consequence for a given datasymbol, such as PAM4 or 4-Level Pulse Amplitude Modulation.
 12. Acircuit according to claim 11 consisting of an analogue filter circuit.13. A circuit according to claim 12 wherein the circuit is implementedin an integrated semiconductor.
 14. A circuit according to claim 11comprising: for each tap a first input port for receiving a firstweight, a second input port for receiving a second other weight, ascaling circuit for scaling the first weight and the second weight, anda weighting circuit for weighting of a signal within the tap to producea tap output, tap output signals from different taps combined to formthe drive signal.
 15. A method comprising: providing a drive current fordriving a component; filtering the drive current with a non-linearfilter to provide pre-compensated drive current pre-compensated fordistortion in a signal resulting from driving the component with thedrive current, wherein an output signal from the component in responseto the pre-compensated drive current has reduced distortion and betterapproximates an ideal transmit signal for an intended modulation.
 16. Amethod according to claim 15 wherein the component comprises a DirectlyModulated Laser (DML).
 17. A method according to claim 16 wherein thedirectly modulated laser comprises a Vertical Cavity Surface EmittingLaser (VCSEL).
 18. A method according to claim 16 wherein the directlymodulated laser comprises a Distributed FeedBack (DFB) laser.
 19. Amethod according to claim 18 wherein filtering is performed with ananalogue filter.
 20. A method according to claim 15 wherein the analoguefilter is implemented in semiconductor.
 21. A method according to claim15 wherein the non-linear filter comprises a non-linear FIR filter. 22.A method according to claim 15 wherein filtering corrects for both arising edge, low to high signal level response, and a falling edge, highto low signal level response.
 23. A circuit comprising: an input portfor receiving a first signal; a plurality of taps, each tap comprisingan input port for receiving a tap input signal, a first input port forreceiving a first weight, a second input port for receiving a secondother weight, and a scaling circuit for scaling an applied weightingbased on the first weight and the second weight to scale the tap signal,the scaled tap signal for modifying the first signal.
 24. A circuitaccording to claim 23 wherein the scaling circuit comprises a switchingcircuit for switching between the different weights to select one weightfor application at a first time and another weight for application atanother time within a same signal to be filtered.
 25. A circuitaccording to claim 23 wherein the scaling circuit comprises a switchingcircuit for switching between the different weights to select one weightfor application at a first time and another weight for application atanother time in dependence upon a content of the signal to be filtered.26. A circuit according to claim 23 comprising a summer for summing anoutput of each of the plurality of taps.
 27. A circuit comprising: aninput port for receiving a first signal; a plurality of taps, each tapcomprising an input port for receiving a tap input signal, a first inputport for receiving a first weight, a second input port for receiving asecond other weight, and a scaling circuit for scaling an appliedweighting between the first weight and the second weight to scale thetap signal, the scaled tap signal for modifying the first signal.
 28. Acircuit comprising: an input port for receiving a first signal; aplurality of taps, each tap comprising an input port for receiving a tapinput signal, a plurality of input ports each for receiving a weight,and a scaling circuit for scaling an applied weighting based on thereceived weights to scale the tap signal, the scaled tap signal formodifying the first signal.
 29. A circuit according to claim 28 whereinthe scaling circuit comprises a switching circuit for switching betweenthe different weights to select one weight for application at a firsttime and another weight for application at another time within a samesignal to be filtered.
 30. A circuit according to claim 28 wherein thescaling circuit comprises a switching circuit for switching between thedifferent weights to select one weight for application at a first timeand another weight for application at another time in dependence upon acontent of the signal to be filtered.
 31. A circuit according to claim28 comprising a summer for summing an output of each of the plurality oftaps.
 32. A circuit comprising: an input port for receiving a firstsignal, the first signal received at a receiver via a communicationinterface and from a remote location; a plurality of taps, each tapcomprising an input port for receiving a tap input signal, a pluralityof weight input ports each for receiving a weight, and a scaling circuitfor scaling an applied weighting based on the received weights to scalethe tap signal, the scaled tap signal for modifying the first signal.33. A circuit according to claim 32 wherein the scaling circuitcomprises a switching circuit for switching between the differentweights to select one weight for application at a first time and anotherweight for application at another time within a same signal to befiltered.
 34. A circuit according to claim 32 wherein the scalingcircuit comprises a switching circuit for switching between thedifferent weights to select one weight for application at a first timeand another weight for application at another time in dependence upon acontent of the signal to be filtered.
 35. A circuit according to claim32 comprising a summer for summing an output of each of the plurality oftaps.
 36. A method comprising providing a receiver for receiving asignal transmitted across an optical fibre and for providing anelectrical first signal; using a filter, filtering the first signal witha non-linear filter to provide compensation to the first signal fordistortion in the signal when transmitted resulting from driving atransmitter at a transmit end, wherein an output signal from the filterbetter approximates an ideal transmit signal for an intended modulation.37. A method comprising: manufacturing a circuit comprising: an inputport for receiving a first signal; a plurality of taps, each tapcomprising an input port for receiving a tap input signal, a pluralityof input ports each for receiving a weight, and a scaling circuit forscaling an applied weighting based on the received weights to scale thetap signal, the scaled tap signal for modifying the first signal;testing the circuit and determining each of the plurality of weightsbased on testing thereof; and setting each of the plurality of weightsbased on a result of the testing thereof and fixing each of theplurality of weights.
 38. A circuit comprising: a non-linear FIR filtercomprising a plurality of taps, each tap having multiple weights and ascaling circuit for scaling the multiple weights to affect a signalpropagating within the tap for nonlinear filtering of a first signal.39. A circuit according to claim 38 wherein the non-linear filter isimplemented as an analogue component within an integrated circuit.